In recent years, various technologies for determining an abnormality in a particulate filter in an internal combustion engine used in a vehicle etc. have been proposed. In a known technology for determining a filter abnormality, as disclosed in Japanese Patent Publication No. 3-38406, an abnormality in a filter is determined based on a comparison of the exhaust gas pressure in the upstream of the filter at a predetermined time after start of filter regeneration and a reference pressure that is determined by the operating state of the engine at that time.
Japanese Patent Application Laid-Open No. 6-323127 discloses a technology of determining an abnormality in a filter by comparing the pressure difference between the upstream and the downstream of the filter just after filter regeneration and the pressure difference in normal time.
Japanese Patent Application Laid-Open No. 6-123216 discloses a technology of determining an abnormality of a filter by determining an expected value of pressure acting on the filter element based on the integrated operation time of the internal combustion engine and the number of engine revolutions at that time and comparing the expected value and an actual pressure.
Japanese Patent Application Laid-Open No. 6-330730 discloses a technology of determining an abnormality of a filter, in which timing for performing the regeneration of the filter is determined based on an estimated amount of trapped particulate matter (particulate matter will be referred to as PM hereinafter) trapped in the filter and also determined based on the integrated number of engine revolutions since the last regeneration, and an abnormality of the filter is determined based on a comparison of these two types of filter regeneration timing. In the technology disclosed in this document, the amount of trapped PM trapped in the filter is estimated using the exhaust gas temperature and the pressure in the upstream and the downstream of the filter as parameters.
In a technology disclosed in Japanese Patent Application Laid-Open No. 7-180528, when the exhaust gas pressure in the downstream of a filter deviates from a predetermined range, it is determined that an abnormality occurs in catalyst or an exhaust passage in the downstream of the filter.
In a technology disclosed in Japanese Patent Application Laid-Open No. 2001-207828, the actual degree of opening of an EGR valve is detected while the degree of opening of an EGR valve is feedback-controlled in such a way that the intake air quantity of the internal combustion engine becomes a target intake air quantity, and when the degree of opening of the EGR valve detected is larger than the degree of opening in the case where the filter is normal, it is determined that the filter is broken.
In a technology disclosed in Japanese Patent Application Laid-Open No. 5-98941, a reflectometer composed of a light source and a light receiving portion is provided in the downstream of a filter, and cracks of the filter are detected based on the reflectivity of light measured by the reflectometer.
In a technology disclosed in Japanese Patent Application Laid-Open No. 8-121150, a sound sensor that senses sounds of a high frequency range is provided in a filter, and clogging and breakage of the filter is detected based on the loudness of the sound sensed by the sound sensor.
When a minor abnormality in which a small amount of PM is allowed to pass through a filter occurs in the filter, the exhaust gas pressure in the upstream of the filter and the pressure difference between the upstream and the downstream of the filter will change slightly, but it is difficult to distinguish such changes from those caused by changes in the amount of the PM trapped in the filter.
Therefore, it is difficult to detect a minor abnormality in a filter by the methods disclosed in Japanese Patent Publication No. 3-38406, Japanese Patent Application Laid-Open No. 6-323127, Japanese Patent Application Laid-Open No. 6-123216, Japanese Patent Application Laid-Open No. 6-330730, Japanese Patent Application Laid-Open No. 7-180528 and Japanese Patent Application Laid-Open No. 2001-207828.
In addition, in the case in which a reflectometer or a sound sensor is provided in the downstream of a filter or in a filter, there is a risk that accuracy of detection by the sensors can be deteriorated by soot adhering to them or by high temperature environment condition under which they are used. Accordingly, it is sometimes difficult for the methods disclosed in Japanese Patent Application Laid-Open No. 5-98941 and Japanese Patent Application Laid-Open No. 8-121150 to detect a minor abnormality in the filter.